Showing papers on "Field electron emission published in 2001"
TL;DR: In this paper, the state of the art of the current research on the electron field emission properties of carbon nanotubes and surveys their ability to provide single or multiple electron sources.
Abstract: Carbon nanotubes, a novel form of carbon discovered in 1991, have been rapidly recognized as one of the most promising electron field emitters ever since the first emission experiments reported in 1995. Their potential as emitters in various devices has been amply demonstrated during the last five years, and recent developments of production techniques are likely to trigger future applications. This report reviews the state of the art of the current research on the electron field emission properties of carbon nanotubes and surveys their ability to provide single or multiple electron sources.
630 citations
TL;DR: In this article, the field emitters comprised of aligned carbon nanotubes are shown to be promising as a primary electron source in an x-ray tube working in a nonultrahigh vacuum ambience.
Abstract: Field emitters comprised of aligned carbon nanotubes are shown to be promising as a primary electron source in an x-ray tube working in a nonultrahigh vacuum ambience. At a pressure of 2×10−7 Torr, the nanotube emitters continue to emit electrons for more than 1 h, and yield better resolved x-ray images than do thermionic emitters, independently of whether the sample is biological or nonbiological. The near-uniformity in energy distribution of electrons emitted from carbon nanotubes might be related to the improved image quality in the field-emission mode.
431 citations
363 citations
TL;DR: In this article, a carbon nanotube paste was prepared and screen-printed to form an electron emission layer on a glass-based substrate for large-area field emission displays.
348 citations
TL;DR: In this article, a triode-type field emission display has been fabricated using carbon nanotube emitters using electrophoresis, and a high brightness of 1000 cd/m2 with uniform emission was obtained at 900 V at the anode and 200V at the gate.
Abstract: A triode-type field emission display has been fabricated using carbon nanotube emitters. Purified single walled carbon nanotubes were selectively deposited onto a cathode electrode in a triode-type structure by an electrophoresis. Emission current was modulated with gate potentials of 100–300 V. A high brightness of 1000 cd/m2 with uniform emission was obtained at 900 V at the anode and 200 V at the gate. The fluctuation of emission current was found to be less than 5% in a fully sealed field emission display. Selective deposition of carbon nanotubes by electrophoresis shows high feasibility for triode-type field emission displays.
263 citations
244 citations
TL;DR: In this article, the authors explore the theory of thin-film low-macroscopic-field (LMF) electron emitters, starting from the need to understand the behaviour of emitters based on amorphous carbon films.
Abstract: Thin flat dielectric films can be low-macroscopic-field (LMF) electron emitters, able to generate electrons when subject to a macroscopic electric field in the range 1–50 V μm −1 . This phenomenon is a known cause of pre-breakdown currents in high-voltage vacuum breakdown, and is now the basis of a broad-area electron-source technology, using carbon-based thin films and other materials. The phenomenon occurs because the dielectric film is, or becomes, an electrically nanostructured heterogeneous (ENH) material, with quasi-filamentary conducting channels between its surfaces. These channels connect to emitting features near or on the film/vacuum surface, or act as electron emitters themselves. The film may contain conducting or semiconducting particles that assist with conductivity and/or act as emitting features. Several forms of thin-film LMF emitter exist: in each case the situation geometry ensures that sufficient field enhancement occurs at the ‘tip’ of the emitting feature for the emission process to be some form of tunnelling field electron emission (probably ‘cold’ in some cases, ‘hot’ in others). This paper explores aspects of the theory of thin-film LMF emission, starting from the need to understand the behaviour of emitters based on amorphous carbon films. A summary review, with extensive references, is given of relevant past work outside the immediate ‘carbon field emission’ context. Relevant aspects of semiconductor field emission theory are noted. Comment is made on the original experiments on diamond field emission, and on theoretical misconceptions in the carbon field emission literature. Analysis of carbon-film emitter behaviour suggests that emission must primarily be due to geometrical field enhancement, that in at least some cases arises from conducting nanostructure inside the film. In one case, published film characteristics can be used to show that sufficient field enhancement should be available. Some problems with an ‘internal field enhancement’ hypothesis are considered and disposed of. Difficulties with Latham’s theory of field-induced emission from ENH materials are pointed out: a new theory, largely qualitative at this stage, can explain longstanding problems: this assumes that dielectric films must be treated as ‘hopping conductors’ not semiconductors. Electron emission takes place via localised surface states: transition to a channel-limited current regime takes place when the surface states no longer have high enough occupation probability to screen the external field, and is accompanied by anomalous band bending at the channel tip. Mathematical theories of band bending and field emission for hopping conductors are required. Some consequences for the design of LMF emitters are noted.
243 citations
TL;DR: In this paper, a well-aligned carbon nanotube array was grown by thermal chemical vapor deposition at 800°C on Fe nanoparticles deposited by a pulsed laser on a porous Si substrate.
Abstract: We have grown well-aligned carbon nanotube arrays by thermal chemical vapor deposition at 800 °C on Fe nanoparticles deposited by a pulsed laser on a porous Si substrate. We also attain a selective growth of carbon nanotubes on a patterned Fe film on Si substrates in terms of pulsed laser deposition and a liftoff patterning method. Field emission measurement has been made on the carbon nanotube (CNT)-cathode diode device at room temperature and in a vacuum chamber below 10−6 Torr. The distance between the CNT cathode and the anode is 60 μm and is kept through an insulating spacer of polyvinyl film. The measured field emitting area is 4.0×10−5 cm2. Our vertically well-aligned carbon nanotube field emitter arrays on the Si-wafer substrate emit a large current density as high as 80 mA/cm2 at 3 V/μm. The transmission electron microscope image shows that they are multiwalled and bamboolike structures and that the tips of some of the carbon nanotube emitters are open. The open tip structure of our CNTs and thei...
220 citations
Patent•
27 Sep 2001TL;DR: An x-ray generating device includes a field emission cathode formed at least partially from a nanostructure (1110) containing material having an emitted electron current density of at least 4 A/cm2 as mentioned in this paper.
Abstract: An x-ray generating device includes a field emission cathode formed at least partially from a nanostructure (1110) containing material having an emitted electron current density of at least 4 A/cm2. High energy conversion efficiency and compact design (1100) are achieved due to easy focusing of cold cathode emitted electron between the cathode (1110) and the gate or anode (1130) and focusing the electron beams at different anode materials (1130), pulsed x-ray radiation with varying energy can be generated from a single device.
218 citations
TL;DR: The work function at the tips of individual multi-walled carbon nanotubes has been measured by an in situ transmission electron microscopy technique as mentioned in this paper, and the tip work function shows no significant dependence on the diameter of the carbon in the range of 14-55 nm.
Abstract: The work function at the tips of individual multiwalled carbon nanotubes has been measured by an in situ transmission electron microscopy technique. The tip work function shows no significant dependence on the diameter of the nanotubes in the range of 14–55 nm. Majority of the nanotubes have a work function of 4.6–4.8 eV at the tips, which is 0.2–0.4 eV lower than that of carbon. A small fraction of the nanotubes have a work function of ∼5.6 eV, about 0.6 eV higher than that of carbon. This discrepancy is suggested due to the metallic and semiconductive characteristics of the nanotube.
213 citations
TL;DR: A template-synthesized polyaniline (PANI) array membrane can be directly used as an electron field emitter, which suggests a new candidate as a field-emitting material as discussed by the authors.
TL;DR: In this paper, a cylindrical field emission diode was proposed to realize a luminescent, mercury-free, tube in a nonplanar geometrical fashion.
Abstract: We show that the field of application of cold electron film emitters can be extended to nonplanar geometries by demonstrating a cylindrical field emission diode. The cathode is a metallic wire on which multiwall carbon nanotubes are grown by the catalytic decomposition of acetylene over a Fe catalyst. The emitter shows excellent performances and can be used to realize a luminescent, mercury-free, tube.
TL;DR: In this article, a comparison between structural, spectroscopic and field-emission properties of aligned and non-aligned multi-wall nanotubes (MWNTs) which are grown by thermal chemical vapour deposition is presented.
Abstract: Carbon-nanotube films are very efficient cathodes for field-emission devices. This study presents a comprehensive comparison between structural, spectroscopic and field-emission properties of films of aligned and non-aligned multi-wall nanotubes (MWNTs) which are grown by thermal chemical vapour deposition. Three types of films are investigated: vertically aligned MWNTs with clean and coated nanotube side walls as well as non-aligned MWNT films. Raman spectra taken on the aligned MWNT films consist of many lines of first-, second- and third-order signals. Several lines are reported here for the first time for MWNTs. The presence of the surface coating leads to a decrease and broadening of the higher-order signals as well as an increase in the disorder-induced contributions in the first-order regime. The aligned MWNT films have excellent field-emission properties with very high emission current densities and low turn-on and threshold fields. The presence of a surface coating has no impact on the efficiency of the field-emission process. Films of non-aligned MWNTs show considerably reduced electron-emission current densities and larger critical fields.
TL;DR: In this paper, ultrananocrystalline diamond (UNCD) films 0.1-2.4 μm thick were conformally deposited on sharp single Si microtip emitters, using microwave CH4-Ar plasmaenhanced chemical vapor deposition in combination with a dielectrophoretic seeding process.
Abstract: Ultrananocrystalline diamond (UNCD) films 0.1–2.4 μm thick were conformally deposited on sharp single Si microtip emitters, using microwave CH4–Ar plasma-enhanced chemical vapor deposition in combination with a dielectrophoretic seeding process. Field-emission studies exhibited stable, extremely high (60–100 μA/tip) emission current, with little variation in threshold fields as a function of film thickness or Si tip radius. The electron emission properties of high aspect ratio Si microtips, coated with diamond using the hot filament chemical vapor deposition (HFCVD) process were found to be very different from those of the UNCD-coated tips. For the HFCVD process, there is a strong dependence of the emission threshold on both the diamond coating thickness and Si tip radius. Quantum photoyield measurements of the UNCD films revealed that these films have an enhanced density of states within the bulk diamond band gap that is correlated with a reduction in the threshold field for electron emission. In additio...
TL;DR: In this paper, the upper limit for the field-emission current that can be obtained from an individual single-walled carbon nanotube was defined, based on the thermally assisted field evaporation of the atoms on the end of the nanotubes and subsequent restructuring of the cap.
Abstract: Extracting large field-emission currents from individual single-walled carbon nanotubes degrades their current–voltage behavior and changes the nanotube structure. Field-emission microscopy observations and field-emission current–voltage measurements revealed behavior characteristic of local heating, followed by thermally assisted field evaporation of the atoms on the end of the nanotube, and subsequent restructuring of the cap. Field evaporation of carbon was found to result in reduction of the length of the tube without catastrophic arcing events. This process defines the upper limit for the field-emission current that can be obtained from an individual single-walled nanotube.
TL;DR: Through first-principles density-functional theory calculations, it is shown that the water-nanotube interaction is weak in zero electric field, but under emission conditions large electric field present at the tube tip increases the binding energy appreciably, thereby stabilizing the adsorbate and lowers the ionization potential.
Abstract: Recent experiments indicate that water molecules adsorbed on carbon nanotube tips significantly enhance field-emission current. Through first-principles density-functional theory calculations we show that the water-nanotube interaction is weak in zero electric field. However, under emission conditions large electric field present at the tube tip: (a) increases the binding energy appreciably, thereby stabilizing the adsorbate; and (b) lowers the ionization potential (IP), thereby making it easier to extract electrons. Lowering of IP is enhanced further through the formation of a water cluster on the nanotube tip.
TL;DR: Although the alkalimetal-doped carbon nanotubes showed high H2 weight uptake, further investigations indicated that some of this uptake was due to water rather than hydrogen, which indicates a potential source of error in evaluation of the storage capacity of doped carbon Nanotubes.
Abstract: Recent progress in the production, purification, and experimental and theoretical investigations of carbon nanotubes for hydrogen storage are reviewed. From the industrial point of view, the chemical vapor deposition process has shown advantages over laser ablation and electric-arc-discharge methods. The ultimate goal in nanotube synthesis should be to gain control over geometrical aspects of nanotubes, such as location and orientation, and the atomic structure of nanotubes, including helicity and diameter. There is currently no effective and simple purification procedure that fulfills all requirements for processing carbon nanotubes. Purification is still the bottleneck for technical applications, especially where large amounts of material are required. Although the alkalimetal-doped carbon nanotubes showed high H2 weight uptake, further investigations indicated that some of this uptake was due to water rather than hydrogen. This discovery indicates a potential source of error in evaluation of the storage capacity of doped carbon nanotubes. Nevertheless, currently available single-wall nanotubes yield a hydrogen uptake value near 4 wt% under moderate pressure and room temperature. A further 50% increase is needed to meet U.S. Department of Energy targets for commercial exploitation. Meeting this target will require combining experimental and theoretical efforts to achieve a full understanding of the adsorption process, so that the uptake can be rationally optimized to commercially attractive levels. Large-scale production and purification of carbon nanotubes and remarkable improvement of H2 storage capacity in carbon nanotubes represent significant technological and theoretical challenges in the years to come.
TL;DR: In this article, anodic aluminum oxide (AAO) templates were fabricated by anodizing Al films and carbon nanotubes (CNTs) were grown by the catalytic pyrolysis of C2H2 at 650°C.
Abstract: Anodic aluminum oxide (AAO) templates were fabricated by anodizing Al films. After the Co catalyst had been electrochemically deposited into the bottom of the AAO template, carbon nanotubes (CNTs) were grown by the catalytic pyrolysis of C2H2 at 650 °C. Overgrowth of CNTs on the AAO templates was observed. The diameter of the CNTs strongly depends on the size of the pores in the AAO template. The electron field emission measurements on the samples showed a turn-on field of 1.9–2.1 V/μm and a field enhancement factor of 3360–5200. Our observation concerning the low turn-on field and high field enhancement factors is explained in terms of a low field screening effect.
TL;DR: In this paper, the authors investigated different types of cathodes operating in an electron diode powered by a high-voltage generator (300 kV, 250 ns, 84 Ω, ⩽5 Hz) and found that the amount of the emitting centers and the time delay in the electron emission were dependent strongly on the accelerating electric field growth rate.
Abstract: We present results of the investigation of different types of cathodes operating in an electron diode powered by a high-voltage generator (300 kV, 250 ns, 84 Ω, ⩽5 Hz). The cathodes which have the same emitting area of 100 cm2 are made of metal–ceramic, carbon fibers, carbon fabric, velvet, or corduroy. We also tested carbon fibers and carbon fabric cathodes coated by CsI. It was shown that for all types of cathodes the electron emission occurs from the plasma which is formed as a result of a flashover of separate emitting centers. The amount of the emitting centers and the time delay in the electron emission were found to depend strongly on the accelerating electric field growth rate. Experimental data concerning the uniformity of the light emission from the cathode surface and divergence of the generated electron beams are presented. Data related to the general parameters of the diode, namely its impedance, power, and energy are given as well. For all the cathodes investigated the observed diode impedance indicated the existence of a quasistationary cathode plasma boundary for electron current density ⩽20 A/cm2. We present the dependencies of the average emitted electron current density and of the time delay in the electron emission on the number of generator shots. We also present data of the vacuum deterioration as a result of the tested cathodes operation. The obtained data are discussed within the framework of plasma formation as a result of cathode surface flashover.
TL;DR: In this article, a triple junction between nanotubes, substrate, and vacuum is proposed to explain the emission characteristics of short and stubby carbon nanotube with diameters of 200 nm and heights of 0.7 μm.
Abstract: Electron emission from vertically aligned carbon nanotubes grown by plasma enhanced chemical vapor deposition has been measured using a parallel plate anode and a 1 μm tungsten probe. The field emission characteristics were measured as a function of the nanotube diameter, length, and areal density. It was found that less densely populated “short and stubby” nanotubes with diameters of 200 nm and heights of 0.7 μm showed the best emission characteristics with a threshold voltage of 2 V/μm and saturation emission current density of 10 mA/cm2. A triple junction between nanotube, substrate, and vacuum is proposed to explain our results.
TL;DR: In this article, an alternative approach that allows a direct measurement of the mechanical and electrical properties of individual nanowire-like structures by in situ transmission electron microscopy (TEM) is presented.
TL;DR: In this article, the surface reconstructions for diamond surfaces in the presence of hydrogen or following thermal annealing are reviewed, and the destructive effects of argon ion bombardment cleaning are discussed.
TL;DR: It is suggested that adjusting the localized electronic states of the emitting regions, by electronic and geometrical means, could improve the field-emission properties of carbon nanotubes.
Abstract: The field-emission mechanism of open-ended single-walled carbon nanotubes (SWNTs) is studied. Owing to electronic effects that directly alter the bonding mode and remarkably influence the work function, an open-ended SWNT has much better field-emission properties than a closed SWNT; owing to geometrical effects that slightly influence the work function and the amplification factor, an open-ended SWNT with relaxation has higher threshold voltage and higher current density compared to one without relaxation. It is suggested that adjusting the localized electronic states of the emitting regions, by electronic and geometrical means, could improve the field-emission properties of carbon nanotubes.
TL;DR: In this article, the influence of an applied electric field on carbon nanotubes protruding from a surface was investigated in situ using a high-resolution scanning electron microscopy, where the authors found that the length of the carbon tubes were shortened after field emission lasted for a period of time.
Abstract: The influence of an applied electric field on carbon nanotubes protruding from a surface was investigated in situ using a high-resolution scanning electron microscopy. Under the applied electric field, the nanotubes flexed to orient themselves parallel to the electric field lines. For moderate field strengths below the electron field emission threshold, the flexed nanotubes relaxed back to their original shapes after the electric field was removed. However, when high electron field emission currents were extracted from the nanotubes, they were permanently deformed, leaving them aligned to the electric field direction after the electric field was removed. For high currents, the length of the carbon nanotubes were found to be shortened after field emission lasted for a period of time.
TL;DR: In this article, a simple thermal chemical vapour deposition method from hydrocarbon molecules, employing the catalytic activity of deposited iron particles, was used to synthesize vertically aligned and nonaligned multi-wall carbon nanotubes.
Abstract: Films of carbon nanotubes are interesting for technical applications
such as cold cathodes in field emission devices. We discuss experiments in
which nanotube films are grown by a simple thermal chemical vapour deposition
method from hydrocarbon molecules, employing the catalytic activity of deposited
iron particles. Using an in situ catalyst preparation method starting
from gaseous Fe(CO)5, films of vertically aligned and
non-aligned multi-wall carbon nanotubes can be synthesised. Nanotube film
growth is discussed as a function of the growth conditions. Steps towards
the formation of horizontally aligned nanotube films and nanotube patterns
are presented. Field emission measurements demonstrate the high electron emission
efficiency of the as-grown films.
TL;DR: In this article, the operation of an integrated gated cathode device using a single vertically aligned carbon nanofiber as the field emission element is described. But, only 1% of the total emitted current is collected by the gate electrode, indicating that the emitted electron beam is highly collimated.
Abstract: We report on the operation of an integrated gated cathode device using a single vertically aligned carbon nanofiber as the field emission element. This device is capable of operation in a moderate vacuum for extended periods of time without experiencing a degradation of performance. Less than 1% of the total emitted current is collected by the gate electrode, indicating that the emitted electron beam is highly collimated. As a consequence, this device is ideal for applications that require well-focused electron emission from a microscale structure.
TL;DR: In this article, a novel approach that relies on electric field induced mechanical resonance for measuring the properties of individual wire-like structures, such as Young's modulus, electron field emission, tip work function, and electrical quantum conductance, is presented.
Abstract: One-dimensional (1D) nanostructures have numerous potential applications in science and engineering. Nanocomposites made of nanowires, such as carbon nanotubes, are likely to decrease material's density and increase its strength, [1] which are of critical importance to space technology. To investigate the uniqueness offered by these materials, new techniques must be developed to quantitatively measure the properties of individual wire-like structures'whose structures are well characterized by electron microscopy techniques, because their properties may sensitively depend on their geometrical shape/configurations and crystal as well as surface structures. Within the framework of in-situ TEM we have recently developed a novel approach that relies on electric field induced mechanical resonance for measuring the properties of individual wire-like structures, such as Young's modulus, electron field emission, tip work function, and electrical quantum conductance. This is a new technique that provides the properties of a single nanowire with well characterized.
TL;DR: The first carbon nanocoil field emitter has been prepared by catalytic thermal chemical vapor deposition and its properties of field emission have been investigated in this paper, where the carbon nanocoils grow selectively from the patterned iron film and maintain their self-organization well.
Abstract: The first carbon nanocoil field emitter has been prepared by catalytic thermal chemical vapor deposition and its properties of field emission have been investigated. The carbon nanocoils grow selectively from the patterned iron film and maintain their self-organization well. The field emission measurement shows that the turn-on field is as low as 180 V at a 130 µm gap. High emission current density, excellent stability, especially uniformity of the field emission from carbon nanocoils have been observed. These properties suggest that the carbon nanocoils is an attractive candidate for the fabrication of flat panel field emission display.
TL;DR: In this article, it was shown that sp2 phase organization plays an important role in the field emission from nanostructured carbons, and that emission is dependent on the cluster size, anisotropy, and mesoscale bonding of sp2 phases.
Abstract: It is shown that sp2 phase organization plays an important role in the field emission from nanostructured carbons. Emission is found to depend on the cluster size, anisotropy, and mesoscale bonding of the sp2 phase, and the electronic disorder. It is found by Raman spectroscopy that increasing the size of sp2 clusters in the 1–10 nm range improves emission. Anisotropy in the sp2 phase orientation can help or inhibit the emission. sp2 clusters embedded in the sp3 matrix or electronic disorder induced by localized defects oriented in the field direction can provide a local field enhancement to facilitate the emission.
TL;DR: In this paper, the effects of O2, Ar, and H2 gases on the field-emission properties of single-walled carbon nanotubes (SWNTs) and multiwalled CNTs (MWNTs).
Abstract: We compare the effects of O2 , Ar, and H2 gases on the field-emission ~FE! properties of single-walled carbon nanotubes ~SWNTs! and multiwalled carbon nanotubes ~MWNTs!. We find that H2 and Ar gases do not significantly affect the FE properties of SWNTs or MWNTs. O 2 temporarily reduces the FE current and increases the turn-on voltage of SWNTs. Full recovery of these properties occurs after operation in UHV. The higher operating voltages in an O 2 environment cause a permanent decrease of the FE current and an increase in the turn-on field of MWNTs. The ratios of the slopes before and after O 2 exposure are approximately 1.04 and 0.82 for SWNTs and MWNTs, respectively. © 2001 American Institute of Physics. @DOI: 10.1063/1.1401785# Considerable interest has been shown of late on the effects of gases on the electronic properties of carbon nanotubes ~CNTs!. 1,2 Oxygen in particular has been found to increase electrical conductance of the nanotubes, with a corresponding increase in the local density of states. The thermoelectric power is also shown to be sensitive to oxygen exposure. 2 While much of the recent work relates to the transport measurements of CNTs little has been done to ascertain the impact of gas exposure as it relates to field emission ~FE!. CNTs have shown great potential as electron field emitters due to the sharpness of their tubes, which enhances the local electric field. 3‐5 The FE current depends strongly on the work function and geometry of the surface, and thus is susceptible to gas exposure. Previous studies have reported about the effects of gases on the FE of CNTs, 6‐9 but these experiments were limited to a single current over a period of gas exposure. Further lacking is a comparative analysis among the various types of CNTs, such as single-walled carbon nanotubes ~SWNTs! and multi-walled carbon nanotubes ~MWNTs!. In this letter, we present a comparative study on the effects of O 2 , Ar, and H 2 on the FE properties of SWNTs and MWNTs. Greater than 90 wt % SWNTs were purchased from Tubes@Rice as a slurry in toluene. 10 The bundles had diameters of 5‐20 nm and consisted of SWNTs having diameters of approximately 1.2 nm. The slurry was deposited onto a conducting Si substrate and allowed to dry. The MWNTs were prepared in our laboratory. A 150 nm layer of Fe was evaporated onto a Si substrate, which was then placed inside a tube furnace. N2 at a flow rate of 250 sccm and H 2 at 11 sccm were introduced into the tube at a pressure of 300 Torr. The sample was then heated to 735 °C for 10 min. The H 2 flow was then turned off and replaced with 20 sccm of C 2H2 for approximately 10 min. The oven and C2H2 flow were then both turned off. The N2 flow rate was set at 300 sccm as the oven cooled to room temperature. The MWNTs had diameters of approximately 25 nm. To measure the FE properties of the CNTs, a positive bias was applied to a spherical platinum anode ~1.1 mm diam! positioned 250 mm away to collect electrons from the grounded nanotubes. The tunneling of electrons through a potential barrier under an applied electric field is described by the Fowler‐ Nordheim ~FN! equation 11